1. Field of the Invention
The present invention relates to a variable magnification optical system and an imaging apparatus. In particular, the present invention relates to a variable magnification optical system that is usable in a video camera, an electronic still camera and the like, and appropriate especially for a surveillance camera, and an imaging apparatus including the variable magnification optical system.
2. Description of the Related Art
Conventionally, surveillance cameras have been used to prevent crimes, to record and the like, and variable magnification optical systems for CCTV (Closed-circuit Television) have been used, as optical systems for surveillance cameras. Such variable magnification optical systems for CCTV need to have compact lens structure and to be producible at low cost. Further, the specifications of the variable magnification optical systems need to be able to cover a high angle-of-view region through a standard angle of view, and the variable magnification optical systems need to be resistant to conditions in indoor and outdoor use. Therefore, two-group zoom lenses that satisfy these requirements, and the structure of which is relatively simple, have been adopted in many cases.
As compact two-group zoom lenses, a two-group zoom lens composed of a positive group and a negative group, and in which positive refractive power precedes, was intensely developed in the era of silver halide film cameras. In such a zoom lens, a positive first group and a negative second group are arranged in this order from an object side. However, in this type of zoom lens, an aperture stop is integrated with the positive first group to make the zoom lens compact, and the length of the entire lens system is short at a wide-angle end, and long at a telephoto end. Inmost surveillance cameras, the size of a casing of the surveillance camera is fixed. Therefore, when this type of zoom lens is applied to a surveillance camera, the lens is arranged at the farthest back position with respect to the casing at a wide-angle end, in which the angle of view is large. Hence, it is necessary to increase the size of the casing, and that causes a serious problem in structure.
Further, since it is difficult to adopt structure in which an aperture stop is variable because of the cost, the diameter of an entrance pupil is constant through variable magnification. Therefore, F-number at a telephoto end is variable-magnification-ratio times larger than F-number at a wide-angle end, and the specifications are not satisfactory as an optical system for a surveillance camera. Further, since the second group has negative refractive power, an angle of a principal ray with respect to a normal to an image plane becomes large at a wide-angle end, and the telecentricity of the lens, which is an important condition for using an electronic imaging device, becomes lower. Since the two-group zoom lens composed of a positive group and a negative group had many inappropriate features, as described above, it was difficult to adopt such a zoom lens for a surveillance camera.
Therefore, two-group structure composed of a negative group and a positive group, and in which negative refractive power precedes, is desirable as a compact two-group variable magnification optical system for a surveillance camera. As a conventional two-group variable magnification optical system composed of a negative group and a positive group, optical systems disclosed, for example, in Japanese Unexamined Patent Publication No. 2001-330773 (Patent Document 1), Japanese Unexamined Patent Publication No. 2001-281543 (Patent Document 2), Japanese Unexamined Patent Publication No. 2007-079108 (Patent Document 3), Japanese Unexamined Patent Publication No. 2007-279335 (Patent Document 4), and U.S. Patent Application Publication No. 20080174880 (Patent Document 5) are known.
Conventionally, a two-group variable magnification optical system composed of a negative group and a positive group, and in which negative refractive power precedes, has been used as a zoom lens for a single-lens reflex camera, for example, such as Patent Documents 1 and 5, which starts from a wide-angle range. However, an example of arranging, as a first lens, a positive lens on the most object side is rare. That is because if the positive lens is arranged as the first lens, the outer diameter and the center thickness of the lens increase, and the size increases more, and that greatly affects the lens, the weight and the cost. Further, since back focus of an entire system is not secured because of spatial restriction by a mirror box unit, such structure has not been adopted very often except in some zoom lens starting from an ultra wide-angle range, and in a lens in which generation of various off-axial aberrations, such as distortion, needs to be suppressed.